Their own art, as well as related Spells and Traps, depict them as alien invaders onto "Primitive Planet Reichphobia" and later "Primeval Planet Perlereino", taking resources and abducting their inhabitants away in canisters (manifested in gameplay as banishing face-down)
You'll see it run in a wide variety of decks, from those that just want to cast giant monsters like Kiora, Sovereign of the Deep to decks that appreciate the multicolor identity of the card, like the various incarnations of Niv-MizzetNiv-Mizzet
This all happens during the declare attackers phase of combat and before blockers are chosen
What have the Pokmon TCG Pocket developers said
There's no break there like "once per turn" it's just all of the time
However, the fact that it can clear the entire field and be summoned with very high attack power means most players wont last